Brillouin light scattering experiments were carried out on porous silicon superlattices with modulation wavelengths in the range 37-167 nm. Phonon frequencies deduced from the Brillouin spectra show good agreement with those obtained from an elastic continuum model for a system with one-dimensional periodicity. Evidence for the existence of a hypersonic phononic bandgap and zone-folded longitudinal acoustic phonons is reported. © 2009 American Institute of Physics. ͓doi:10.1063/1.3275742͔It is well known that the introduction of artificial spatial periodicity in the elastic properties of a materials system results in Brillouin zone-folding. Such folding is often accompanied by the appearance of bandgaps in the phonon frequency spectrum and thus materials systems that exhibit this phenomenon are promising candidates for phononic crystals. Porous silicon ͑-Si͒ is especially interesting in this regard since its elastic properties can be varied over nanometer length scales and because it can be readily integrated into existing silicon technologies.In this letter, the results of Brillouin scattering experiments on -Si superlattices ͑SLs͒ are reported. Good agreement is obtained between measured phonon frequencies and those calculated from a one-dimensional elastic continuum model. Clear evidence for the existence of an acoustic bandgap and for zone-folding of longitudinal acoustic phonons is presented.Two sets of -Si SLs and associated single layer films were synthesized by anodization of p + -type ͑100͒-oriented crystalline silicon ͑5-20 m⍀ cm͒ in an electrolyte composed of 1͑49% HF͒:1͑C 2 H 5 OH͒. Each set was made using a different supply of HF with one solution yielding single layer films with gravimetrically-determined porosities of 0.59 ͑J = 149 mA/ cm 2 ͒ and 0.52 ͑J = 101 mA/ cm 2 ͒, while the other resulted in films with porosities of 0.56 ͑J = 149 mA/ cm 2 ͒ and 0.46 ͑J = 101 mA/ cm 2 ͒. The difference in film porosity for the same applied current density is ascribed to slight differences in HF concentration in the two solutions. The single layer films were ϳ5 m thick. SLs with a binary periodic variation in porosity were formed by alternating the current density between the above values. Etching times were chosen so that the constituent layer thicknesses, d 1 and d 2 , were nearly equal ͓see Fig. 1͑a͔͒. SLs with modulation wavelengths of D = d 1 + d 2 from 37 nm to 167 nm were made for the Brillouin experiments. The porosities, refractive indices, and elastic properties of the SL constituent layers were taken to be the same as those of single layer films formed under identical conditions. It is noted that images like that of Fig. 1͑a͒ were not obtained for SLs with D Ͻ 500 nm. Indirect evidence of prescribed SL formation, however, was provided by measurements of overall SL film thickness ͑ϳ5 m͒ which was within 10% of the value calculated using known etch parameters.Brillouin scattering experiments were carried out in air at room temperature using a backscattering geometry ͓see Fig. 1͑b͔͒. Incident light o...

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Measurements of contact angle of ethanol-water solutions were performed on crystalline silicon and on mesoporous silicon films with porosities up to ∼72%. Water contact angles of 44 • and 76 • were measured for untreated and HF-dipped crystalline silicon, respectively, consistent with previous studies. The contact angle for ethanol-water mixtures was found to decrease with increasing ethanol concentration for both untreated crystalline silicon and also for HF-dipped crystalline silicon up to an ethanol concentration of ∼80%; at higher concentrations the contact angle approached zero. Similar behaviour was observed on mesoporous silicon surfaces for ethanol concentrations 40%, above which the contact angle dropped abruptly to an immeasurably small value. This behaviour is attributed to a decrease in surface tension with increasing ethanol concentration. For all ethanol-water solutions, a minimum value of contact angle was observed at a porosity of ∼40%, above which it remained approximately constant. The behaviour of contact angle with porosity can be explained by a change in the Wenzel roughness parameter due to changes in the specific surface area of the film.

Brillouin light scattering experiments were done on porous silicon-based optical Bragg mirrors with modulation wavelengths of ∼100 nm. By using a combination of pseudo-reflection and backscattering geometries, phonon dispersion curves along the superlattice modulation axis were mapped. Excellent agreement is obtained with the bulk acoustic mode band structure calculated using a one-dimensional elastic continuum model. In addition to zone-folding of the bulk longitudinal mode dispersion curve, the samples are marked by a surface-localized acoustic mode at the superlattice-air interface. The frequency of this mode lies near the upper edge of a phononic band gap centered at ∼16 GHz. These results, along with optical reflectance data showing visible-range photonic band gaps, reveal that these samples are one-dimensional hypersonic phononic-photonic crystals.

Brillouin spectroscopy was used to study surface acoustic waves on a supported layer of (111)-oriented porous silicon having a thickness of 2.7 μm and a porosity of 30%. The Rayleigh surface wave velocities were found to be significantly lower than corresponding velocities for crystalline silicon. A complete set of elastic constants for the porous layer was determined from the measured directional dependence of the surface wave velocity in the (111) plane. The best-fit constants are C11=56.0±0.7 GPa, C12=6.7±0.3 GPa and C44=37.0±0.8 GPa. The anisotropy factor, η=1.50 indicates that the porous layer is elastically anisotropic.